Such a process is for example known from EP-A-0288253, in which, as in other publications, use is made of bis-N-acyl lactams having the formula: ##STR1##
in which A=alkyl or an aromatic group and n is generally between 3 and 11.
The bis-N-acyl lactams used in the examples are generally tere- or isophthaloyl bislaurocaprolactam or biscaprolactam.
These bis-N-acyl lactams however have the drawback of a relatively low reaction rate, as a result of which long reaction times are required to realize the desired increase in molecular weight, which may lead to undesired side-reactions, e.g. discolouration of the polyamide or polyester.
In practice, bisoxazolines or bisoxazines are therefore more preferably used as chain extenders for polyamides. The drawback of these is however that they react with the polyamide's carboxylic end groups, as a result of which the polyamide obtained contains excess amino end groups and the polyester excess hydroxyl endgroups, which adversely affects the thermal oxidative stability.